1. Field of the Invention
The present invention relates to a touch screen, and more particularly, to a signal conversion control circuit applied to a touch screen and the method thereof.
2. Description of the Related Art
A touch screen is a display control apparatus, which can detect touch positions and touch strength from a user. Because the control method of the touch screen is more intuitive than that of a normal mouse and keyboard, it provides an easier use of a computer operational interface for non-traditional users. Therefore, most early applications of touch screens have been in public service devices, e.g., computer-assisted instruction (CAI) terminals, demo display and ATM machines, as a laborless machine. With the progress of the technology, people have more demands in intuitive machines than ever, and therefore the touch screens are more widely used on mobile machines, such as mobile telephones, personal digital assistants, and handheld game machines.
There are a variety of detection methods for a touch screen. Generally, the detection methods include resistor-type, capacitor-type, infrared-type optical-type, and others. The resistor-type touch screen determines the touch positions or touch strength in accordance with the resistor voltage divider on the touch screen. In addition, in light of the resistor and wiring arrangement, the resistor-type touch screen can be divided into four-wired, five-wired to eight-wired designs.
FIG. 1 shows a traditional signal conversion control circuit applied to a resistor-type touch screen. The signal conversion control circuit 100 is connected to a touch screen 500, and includes an analog-to-digital converter (ADC) 102, a first switch 104, a second switch 106 and a sample-and-hold circuit 108. The touch screen 500 can be a four-wired or five-wired resistor-type touch screen, and includes resistors 502, 504 and 506. The first switch 104 connects a supplied voltage to the resistor 502. The second switch 106 connects a ground potential to the resistor 504. The resistor 506 connects the resistors 502 and 504 to the input terminal of the sample-and-hold circuit 108. The input terminal of the ADC 102 is connected to the output terminal of the sample-and-hold circuit 108, while the high-level reference voltage terminal thereof is connected to the supplied voltage and the low-level reference voltage terminal is connected to the ground potential.
In a sampling mode, the first switch 104 and the second switch 106 are turned on to connect the supplied voltage and ground potential to the touch screen 500. The touch screen 500 generates a divided voltage of the supplied voltage as an output signal through a voltage divider formed by the resistors 502 and 504. After the output signals are completely sampled through the third sample-and-hold circuit 108, the flow enters a signal translation mode. In the meantime, the first switch 104 and the second switch 106 are turned off. The ADC 102 converts the sampled-and-held output signals of the touch screen 500 in accordance with the sampled-and-held supplied voltage and ground potential connected by the high-level and low-level reference voltage terminals.
In the signal conversion mode, the first switch 104 and the second switch 106 are turned off, and the current flowing through the touch screen 500 in the sampling mode can be reduced. The resistance combined by the resistors 502 and 504 is approximately in the range of 200 to 900 ohms, and the supplied voltage is approximately 5 volts. Therefore, about 5.6 to 25 mA current can be reduced. However, the voltage across a turned-on first switch 104 and a turned-on second switch 106 is about 0.3 volts, therefore, the maximal output signal of the touch screen is limited to 4.7 volts. In considering that the difference of the high-level and low-level reference voltages is about 5 volts, the accuracy of the signal conversion for the ADC 102 is significantly affected.
FIG. 2 shows another traditional signal conversion control circuit applied to a resistor-type touch screen. The signal conversion circuit 200 is connected to the touch screen 500, and includes an analog-to-digital converter (ADC) 202, a first switch 204, a second switch 206 and a sample-and-hold circuit 208. The first switch 204 connects a supplied voltage to the resistor 502 and a high-level reference voltage terminal of the ADC 202. The second switch 206 connects a ground potential to the resistor 504 and a low-level reference voltage terminal of the ADC 202. The resistor 506 connects the junction between the resistors 502 and 504 to the input terminal of the sample-and-hold circuit 208. The signal input terminal of the ADC 202 is connected to the output terminal of the sample-and-hold circuit 208.
In the sampling mode, the first switch 204 and the second switch 206 are turned on to connect the supplied voltage and ground potential to the touch screen 500. The touch screen 500 uses the divider circuit formed by the resistors 502 and 504 to generate a divided voltage of the supplied voltage as the output signal thereof. After the sample-and-hold circuit 208 completes the sample-and-hold action on the output signals, the flow enters a signal conversion mode. Meanwhile, the first switch 204 and the second switch 206 are still turned on, and the ADC 202 converts the sampled signals in accordance with the supplied voltage and ground potential connected by the high-level and low-level reference voltage terminals.
As shown in FIG. 2, the range of input signals of the ADC 202 is almost the same as the voltage difference between the high-level and low-level reference voltages, about 4.7 volts. Therefore, there is no accuracy problem when the signal conversion control circuit 100 performs. However, in the signal conversion mode, the first switch 204 and the second switch 206 are still turned on, and the current flowing through the touch screen 500 is about 5.6 mA to 25 mA, significantly increasing the power consumption of the signal conversion control circuit 200.
Therefore, it is necessary to design a signal conversion control circuit applied to the resistor-type touch screen which is capable of retaining the advantages of the above two prior arts and also removing the drawbacks of the same two prior arts.